This invention generally relates to cathode ray tubes and, particularly, to an improved means for damping vibrations in such tubes having a tension mask.
As is known in the art, a cathode ray tube is generally constructed of a glass envelope and includes an electron gun located within a neck portion of the envelope for generating and directing three electron beams to the screen of the tube. The screen is located on the inner surface of a faceplate panel of the tube and is made up of an array of elements of three different color emitting phosphors. A color selecting electrode, which may be either a shadow mask or a focus mask, is interposed between the gun and the phosphor screen to permit each electron beam to strike only the phosphor elements associated with that beam. Each electron beam is scanned by an electromagnetic deflecting device for impingement on a desired phosphor of the phosphor screen.
In conventional color cathode ray tubes having two-dimensionally curved color selecting electrodes or shadow masks, the curvature of the mask and its thickness causes it to be structurally self-supporting. Another type of commercial shadow mask is tensioned on a support frame and is not self-supporting as is the two-dimensionally curved type. The tension shadow mask contains a plurality of very thin parallel vertically extending strands maintained at high tension. In another type of tension mask, the frame supporting the mask is designed to permit the mask to de-tension during thermal treatment of the tube. The afore-described cylindrical tension shadow mask configurations are prone to vibrations, as may be caused by external mechanical pulses, or by a speaker in an associated television receiver, for example. The resonant frequency of vibration of the mask will vary depending on the mechanical parameters of and tension in the mask. Any vibration of the mask will cause electron beam landings to be out of registry with their respectively associated phosphor elements, causing color impurities in the reproduced images.
Various means have been suggested for damping the resonant vibrations described above. One example for damping the vibration of a tension mask includes damping wires stretched across the mask to damp vibrations in the mask strands by relative motion between the strands and the wires. The damping wires can be held against the mask strands because of the curved nature of the mask. The ends of the wires are secured to the frame supporting the tension mask by tabs which hold the wires under light tension. With such an arrangement, the strands are resiliently pressed by the wires and, therefore, are not likely to vibrate by external mechanical shocks or electron beam bombardment. Disadvantages inherent in a mask assembly of this type include variations in the height of the tabs which could either cause the wires not to touch the strands or press on them to cause noticeable deflection of the strands so as to prevent damping of their motions. The problem is exacerbated by the use of tension masks having specific tension distributions across the mask or in de-tension mask frames resulting in relatively low strand stretching forces.
This invention is directed to providing a solution to the problem of damping resonant vibrations in a tension shadow mask and thus avoiding a deterioration of picture quality caused by external vibrations.
The present invention provides a cathode ray tube having a color selection electrode tension mask attached to a support frame. The tension mask includes damper wire support springs attached to, and extending from, opposite sides of the tension mask support frame. The damper wire support springs having a compliance section supporting a damper wire in contact with and across the surface of the tension mask for damping vibrations in the mask.